1. Field of the Invention
The present invention relates to a voice coil type lens drive assembly for adjusting the focus of a lens by utilizing a repulsive magnetic force which a magnetic field created upon energization of a coil induces between the coil and a fixed magnet opposed to the coil.
2. Description of the Related Art
Heretofore, in connection with a voice coil type lens drive assembly for adjusting the focus of a lens by utilizing a repulsive magnetic force which a magnetic field created upon energization of a coil induces between the coil and a fixed magnet opposed to the coil, a spring for returning a movable unit to an initial position has been known as is disclosed, for example, in Patent Literatures 1 and 2.
In the spring disclosed in Patent Literature 1 (Japanese Patent Laid-Open Publication No. 2006-201525, see FIG. 1 and paragraph 0031), as shown in FIG. 17, thin spring wires 125 are formed between an inner periphery-side end 109a and an outer periphery-side end 109b to constitute elastic portions.
In the spring disclosed in Patent Literature 2 (Japanese Registered Utility Model No. 312606, see FIG. 3), as shown in FIG. 18, the outside of a spring 150 is attached to a fixed frame, while the inside of the spring 150 is attached to a movable unit, and U-bent portions 53 are formed therebetween to constitute elastic portions.
Although the above conventional springs do not involve any functional effect, it is desired to remedy the following inconvenience.
In the spring disclosed in Patent Literature 1, as shown in FIG. 17, in case of forming elastic portions 125, 131, 128 and 130, since the elastic portions are formed between the inner periphery-side end 109a and the outer periphery-side end 109b, an outline of the entire assembly becomes large and it is difficult to apply the construction in question to a downsized lens drive assembly.
The spring 150 disclosed in Patent Literature 2, which is shown in FIG. 18, has been proposed to improve the elastic portions of the spring disclosed in Patent Literature 1. In the spring 150, U-bent portions 153 are formed between the outside and the inside to constitute elastic portions by utilizing narrow spaces formed between four square corners and a circular lens barrel. Therefore, the paths of the elastic portions are short, resulting in that the spring elastic modulus is large and this spring characteristic causes a large change in the lens controlling drive force. This is not desirable and gives rise to the problem that a higher accuracy than necessary is required for controlling the lens position.
The above conventional constructions further involve the problem that when the lens moves against the bias of the spring, there occurs an offset in a peculiar direction due to characteristics induced by the shape of the spring. That is, when the spring expands or contracts, the lens moves while being pulled in the expanding or contracting direction, i.e., inwards or outwards, with the result that tilting of the lens is apt to occur.
More particularly, in the construction disclosed in Patent Literature 1 and shown in FIG. 17, the positions of connections 128 and 130 are different in the circumferential direction, so that in operation, a force acting in a rotational direction is exerted on the lens. In paragraph 0031 of Patent Literature 1 it is described that the arcuate connections 128 and 130 are each in an arcuate shape of a half or more of a circle and are therefore easy to undergo an elastic deformation and absorb stress. Actually, since the portions in question are close to both inner and outer peripheries, there is the problem that forced oscillation is apt to occur due to deformation of the portions in question and that with the resulting resonance phenomenon, the oscillation in the rotational direction is difficult to be damped.
In the case of the spring 150 disclosed in Patent Literature 2 and shown in FIG. 18, the lens is pulled in an expanding or contracting direction of the spring and hence tilting of the lens is apt to occur. Consequently, a frictional force acts on the sliding portion as the lens moves, and when the magnetic driving force becomes weak at the end of the movement, it undesirably ends up with a larger elastic modulus.
That is, a lens unit starts operating from a position offset in a different direction, depending on in what attitude the photographing device concerned is used. Just after energization of a coil and hence start of operation, the motion of the lens system involves a peculiar oscillation under the influence of the magnetic force created between the magnet and coil as a drive source, a relative distance between the two or a coefficient of friction with a contacted slide portion at an offset position.
The oscillation system of the lens unit differs depending on the moving speed and weight of the lens unit, so in order to attenuate the turning motion of the lens unit quickly in correspondence to a change in damped state of the resonance phenomenon of the lens system it is necessary to change the state of absorption of the oscillation system.